Electrohydraulic servovalves are well known. These may be thought of as having either a single stage, or as having multiple stages. In both forms, a valve spool is slidably mounted within a cylinder for controlled movement therealong. When the valve spool is in a centered or null position within the cylinder, various lands on the valve spool cover ports that communicate with the control outlets to prevent flow through the valve. The direction and magnitude of spool movement off-null controls the flows through the valve. Various forms of single-stage servovalves are representatively shown and described in U.S. Pat. No. 4,951,549, U.S. Pat. No. 5,263,680, U.S. Pat. No. 4,641,812 and U.S. Pat. No. 5,146,126, the aggregate disclosures of which are hereby incorporated by reference.
A single-stage or direct-drive valve generally has a motor or some other mechanism that directly engages the valve spool, and which selectively causes the spool to move off-null. A multiple-stage valve may have a pilot stage that controls movement of a valve spool in a second stage. The pilot stage may be an electrical section, and the second stage may be a hydraulic section. One example of a two-stage electrohydraulic servovalve is shown and described in U.S. Pat. No. 3,228,423 A, the aggregate disclosure of which is hereby incorporated by reference.